MEXICO CITY—A reinterpretation of data concerning the first rotavirus vaccine, RotaShield, whose manufacturer withdrew it from the market in 1998, has some of the world's rotavirus experts gathered at a conference here wondering whether the risks associated with the vaccine were far less serious than originally believed.

In July of 1999, the Centers for Disease Control issued an alert about what appeared to be a statistically significant increase in intussusception—a potentially dangerous condition in which the small intestine collapses in on itself—that occurred in the days following vaccination. A cluster of 15 cases of intussusception was identified in infants who recently had the vaccine.

According to data analyzed by Lone Simonsen, a National Institutes of Health researcher, more than 80% of all cases of intussusception events that were associated with the vaccine happened with babies who received the vaccine after 4 months of age, which was at the far...

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